1. Field of the Invention
The present invention relates to a solid-state image pick-up device in which photoelectric converting films to be a plurality of layers are laminated on a semiconductor substrate in which a signal reading circuit is formed, and more particularly to a solid-state image pick-up device of a photoelectric converting film lamination type in which each of signal charges generated in the photoelectric converting film to be each layer is uniformly moved toward a semiconductor substrate side, thereby suppressing the generation of an afterimage.
2. Description of the Related Art
A prototypical element of the solid-state image pick-up device of a photoelectric converting film lamination type has been described in JP-A-58-103165, for example. The solid-state image pick-up device has such a structure that three photosensitive layers are laminated on a semiconductor substrate and each of electric signals for red (R), green (G) and blue (B) colors detected in the photosensitive layers is read by an MOS circuit formed on a surface of a semiconductor substrate.
While the solid-state image pick-up device having such a structure was proposed in the past, there have remarkably been progressed an image sensor of a CCD type and an image sensor of a CMOS type in which a large number of light receiving portions (photodiodes) are integrated on a surface portion of a semiconductor substrate and color filters for red (R), green (G) and blue (B) colors are laminated on each of the light receiving portions. At present, an image sensor having several millions light receiving portions (pixels) integrated on one chip is loaded onto a digital still camera.
Referring to the image sensor of the CCD type and the image sensor of the CMOS type, however, their technical progress has been developed up to an almost limit and an opening of a light receiving portion has a size of approximately 2 μm which is close to a wavelength order of an incident light. Consequently, there is a confronted problem in that a manufacturing yield is poor.
An upper limit of the amount of photoelectric charges to be stored in the light receiving portion which is microfabricated is small, that is, has approximately 3000 electrons. Consequently, it is also hard to clearly represent 256 gradations. For this reason, it is difficult to expect a more excellent image sensor of a CCD type or a CMOS type in respect of picture quality and a sensitivity.
As a solid-state image pick-up device to solve these problems, therefore, the solid-state image pick-up device proposed in JP-A-58-103165 has been reconsidered, and image sensors described in Japanese Patent No. 3405099 and JP-A-2002-83946 have been proposed newly.
The image sensor described in the Japanese Patent No. 3405099 has such a structure that ultrafine particles of silicon are dispersed into a medium to form a photoelectric converting layer, and three photoelectric converting layers in which a size of the ultrafine particle is varied are laminated on a semiconductor substrate and an electric signal corresponding to an amount of receipt of a light for each of red, green and blue colors is generated from each of the photoelectric converting layers.
Also in the image sensor described in the JP-A-2002-83946, similarly, three nanosilicon layers having different particle sizes are laminated on the semiconductor substrate and each of electric signals for red, green and blue colors detected from each of the nanosilicon layers is read into a charge storage diode formed in a surface portion of the semiconductor substrate.
FIG. 6 is a typical sectional view corresponding to two pixels in the related-art solid-state image pick-up device of a photoelectric converting film lamination type. In FIG. 6, a surface portion of a P well layer 1 formed on an n-type silicon substrate is provided with an impurity region 2 having a high concentration for storing a read signal, an MOS circuit 3 for reading the read signal, an impurity region 4 having a high concentration for storing a green signal, an MOS circuit 5 for reading the green signal, an impurity region 6 having a high concentration for storing a blue signal, and an MOS circuit 7 for reading the blue signal.
The MOS circuits 3, 5 and 7 are constituted by impurity regions for a source and a drain which are formed on the surface of the semiconductor substrate, and a gate electrode formed through a gate insulating film 8. An insulating film 9 is laminated on the gate insulating film 8 and the gate electrode and is flattened, and a shielding film 10 is laminated thereon. The shielding film is formed by a metallic thin film in many cases. For this reason, an insulating film 11 is further formed thereon.
Signal charges stored in the impurity regions 2, 4 and 6 having a high concentration for storing color signals are read to an outside by means of the MOS circuits 3, 5 and 7.
A pixel electrode film 12 divided for each pixel is formed on the insulating film 11 shown in FIG. 6. The pixel electrode film 12 for each pixel is conducted to the impurity region 2 having a high concentration for storing a red signal every pixel through a columnar contact electrode 13. The contact electrode 13 is electrically insulated from portions other than the pixel electrode film 12 and the impurity region 2 having a high concentration.
A photoelectric converting film 14 for detecting a red color is laminated on each pixel electrode film 12 in a one-sheet structure in common to each pixel, and furthermore, a transparent common electrode film 15 is formed thereon in a one-sheet structure in common to each pixel.
Similarly, a transparent insulating film 16 is formed on the common electrode film 15 and a transparent pixel electrode film 17 divided for each pixel is formed thereon. Each pixel electrode film 17 and the corresponding impurity region 4 having a high concentration for storing a green signal for each pixel are conducted through a columnar contact electrode 18. The contact electrode 18 is electrically insulated from portions other than the pixel electrode film 17 and the impurity region 4 having a high concentration. A photoelectric converting film 19 for detecting a green color is formed on each pixel electrode film 17 in a one-sheet structure in the same manner as the photoelectric converting film 14, and a transparent common electrode film 20 is formed thereon.
A transparent insulating film 21 is formed on the common electrode film 20 and a pixel electrode film 22 divided for each pixel is formed thereon. The pixel electrode film 22 is conducted to the corresponding impurity region 6 having a high concentration for storing a blue signal for each pixel through a columnar contact electrode 26. The contact electrode 26 is electrically insulated from portions other than the pixel electrode film 22 and the impurity region 6 having a high concentration. A photoelectric converting film 23 for detecting a blue color is laminated on the pixel electrode film 22 in a one-sheet structure in common to each pixel and a transparent common electrode film 24 is formed thereon, and a transparent protective film 25 is formed as an uppermost layer.
When a light is incident on the solid-state image pick-up device, photoelectric charges corresponding to the amount of an incident light for each of the blue, green and red colors are excited in each of the photoelectric converting films 23, 19 and 14, and a voltage is applied between the common electrode films 24, 20 and 15 and the pixel electrode films 22, 17 and 12. Consequently, the respective photoelectric charges flow into the impurity regions having a high concentration 2, 4 and 6 and are read as blue, green and red signals to the outside through the MOS circuits 3, 5 and 7.
In the related-art solid-state image pick-up device of the photoelectric converting film lamination type shown in FIG. 6, the signal charges generated in the photoelectric converting films 14, 19 and 23 flow to the signal charge storage portions (the impurity regions having a high concentration) 2, 4 and 6 through contact electrodes 13, 18 and 26 respectively, and are read through the MOS circuits 3, 5 and 7. A length of the contact electrode 13 connected to the pixel electrode film 12 provided on the photoelectric converting film 14 to be a lower layer is smaller than that of the contact electrode 18 connected to the pixel electrode film 17 of the photoelectric converting film 19 to be an intermediate layer, and a length of the contact electrode 26 connected to the pixel electrode film 22 of the photoelectric converting film 23 to be a higher layer is greater than that of the contact electrode 18. For this reason, resistance values of the contact electrodes 13, 18 and 26 are in order of the contact electrode 13<the contact electrode 18<the contact electrode 26.
When the resistance values of the contact electrodes 13, 18 and 26 are different from each other, a time required for moving the signal charges generated in the photoelectric converting films 14, 19 and 23 to the electric charge storage portions 2, 4 and 6 become nonuniform. When the signal charge generated in the photoelectric converting film 14 to be a lower layer is read, the signal charges generated in the photoelectric converting films 19 and 23 to be the intermediate and higher layers remain in the photoelectric converting films and the contact electrodes and are read together when the signal charges are to be subsequently read. In other words, there is a problem in that an afterimage phenomenon is generated.